Safety and immunologic response of a viral vaccine to prostate-specific antigen (PSA) in combination with radiation therapy when metronomic-dose interleukin 2 (IL-2) is used as an adjuvant. We have previously reported on the safety and immunologic response of a poxvirus-based vaccine encoding PSA used in combination with radiation therapy in patients with localized prostate cancer. We hypothesized that a metronomic dose of IL-2 as a biological adjuvant would cause less toxicity while maintaining immunologic response. Eighteen patients with localized prostate cancer were treated in a single arm trial using previously established doses of vaccine and radiation therapy. The vaccine used was a recombinant vaccinia virus engineered to encode PSA admixed with a recombinant vaccinia encoding the costimulatory molecule B7.1, followed by booster vaccinations with a recombinant fowlpox vector expressing PSA. Patients received a total of eight planned vaccination cycles, once every 4 weeks, with granulocyte-macrophage colony-stimulating factor given on days 1to 4 and IL-2 at a dose of 0.6MIU/M2 given from days 8 to 21 after each vaccination. Definitive external beam radiation therapy was initiated after the third vaccination cycle. Patients were evaluated for safety and immunologic response. Toxicity and immunologic activity were compared with the previously reported regimen containing a higher dose of IL-2. Seventeen of 18 patients received all eight cycles of vaccine with IL-2. Five of eight HLA-A2+ patients evaluated had an increase in PSA-specific T cells of &#8805;3-fold. Toxicities were generally mild, with only seven vaccination cycles of 140 given resulting in grade 3 toxicities possibly attributable to IL-2. Metronomic-dose IL-2 in combination with vaccine and radiation therapy is safe, can induce prostate-specific immune responses, and has immunologic activity similar to low-dose IL-2, with markedly reduced toxicities.